In the high production of metal rings, such as automobile rims, a flat strip of metal is formed into circular shape with a gap between end surfaces, and such surfaces are joined by means of resistance flash welding.
Due to physical variations between one run of metal strips and another, the gaps between end surfaces of the rolled rims cannot be precisely controlled during the rim-forming operation and, because of the economics required in high production manufacturing, the gaps cannot be redimensioned or measured before the flash welding operation. In a closely-controlled operation, most rim gaps are held to acceptable tolerances but, as above noted, physical variations in the metal may cause gaps that are too large for acceptable welds.
If a rim is formed so that no gap exists between its end surfaces, such surfaces will "freeze" at the start of flashing function of the machine and no effective flashing will occur and no satisfactory weld will be made. However, a control circuit, such as disclosed in U.S. Pat. No. 3,542,994, issued to me and Richard L. Curtner in Nov. 24, 1970, and assigned to the assignee of the present invention, is effective to prevent "freezing," even if the adjoining ends of a rim are in touching relation. The difficulty occurs when a gap larger than normal is formed, and applicant is unaware of any prior art which will provide for an acceptable weld under such gap conditions.
In the manufacture of automobile rims, metal strips of predetermined length are sheared from a long length of fastmoving continuous strip. The sheared strips are rolled to rim shape at high speeds and are individually loaded into a flash welder, welded and thereafter ejected from the welder in high production manner. Thus it is impractical to measure the gap in a rolled rim and either reject or redimension the gap thereof in this high production line. Yet, the welds in the individual rims must be satisfactory or the rims will fail, either in subsequent forming operations, or in actual use.
My invention provides means whereby when a ring is loaded into a flashwelder with a gap too large to produce a suitable weld, the welding operation is either aborted, or a signal is given, or both. To obtain this desirable result, the movement of the movable platen toward the stationary platen is compared with its position when flashing actually begins. If the gap is within tolerance, the movable platen will be in the correct position at the time of welding. On the other hand, if the gap is too large, the movable platen will move beyond its position for flashing before the end surfaces of the rim abut and flashing begins, and it is this comparison of platen movement with the start of flashing that I utilize to determine whether or not conditions are proper for an acceptable weld.